This invention relates to communications in an earth borehole and, more particularly, to a wireless telemetry system and method for communication in a cased borehole in which tubing is installed. The invention further relates to the communication of information in such a system, in close to real time, during perforation, testing, stimulation (such as fracturing) and production.
During perforation, testing, stimulation, treating, and/or production of a well, it would be very advantageous to have accurate information concerning conditions downhole; particularly, conditions such as pressure, temperature, fluid flow rate, weight on a packer, etc. Techniques for utilizing information concerning these conditions have advanced in recent years. Accordingly, if suitable information concerning downhole conditions is available, the interpretation resulting therefrom can be used to make decisions that can greatly enhance the ultimate production and cost efficiency of the well. An example is the so-called Nolte-Smith technique for interpretation of fracturing pressures (see "Interpretation Of Fracturing Pressures", Nolte et al., SPE, 1981), which is widely used in industry, and has intensified the desire for continuous bottom-hole pressure data. The importance of obtaining these data as they occur (in close to real time), for example for controlling a fracturing operation, is substantial (see, for example, "The Real-Time Calculation Of Accurate Bottomhole Fracturing Pressure From Surface Measurements", R. H. Hannah et al., SPE, 1983; "Prediction Of Formation Response From Fracture Pressure Behavior", M. W. Conway et al., SPE, 1985; "Computerized Field System For Real Time Monitoring And Analysis Of Hydraulic Fracturing Operations", M. P. Cleary et al., SPE, 1986). However, to Applicants' knowledge, there is no currently existing technique for obtaining measurements of downhole conditions that does not have significant drawbacks.
Among the existing techniques for obtainment of data on downhole conditions with tubing in place, are the following:
1. Data can be taken with a measuring instrument downhole, and recovered after completion of the job. This has the obvious drawback of the unavailability of the data during the job, and limitations on downhole power and data collecting ability.
2. In a situation of a packerless completion, the bottom hole pressure can be estimated at the surface via measurement of the annular static fluid column. This provides only a low frequency filtered pressure measurement. Also, the casing is exposed to treating pressures.
3. Bottom-hole conditions can be approximated from conditions measured uphole, for example pressure, fluid properties, etc. However, the accuracy of these indirect measurements is generally poor. Among the reasons, is the close proximity to surface pumping noise.
4. Sensing devices can be placed downhole with an electrical cable strapped to the outside of tubing, or run inside the tubing, or can be lowered after the fact to connect downhole or to interrogate a downhole device. These techniques have obvious advantages in providing a good communications link. However, in addition to the cost of the cabling, the possibility of the cable tangling, interfering with mechanical structure and/or fluid flow, breaking, or not making suitable contact downhole, renders this technique less than ideal in many applications.
The prior art describes a variety of wireless communications systems for measurement while drilling. Some of these are measurement-while-drilling systems that utilize the drill pipe and the formations (and/or metal casing, to the extent present) to transmit electromagnetic signals over a "transmission line" that includes the drill string as a central conductor, and the formations (and/or casing, as the case may be) as outer conductors.
In the U.S. Pat. No. 4,057,781 Scherbatskoy, there is disclosed a measurement and communications system for measurement while drilling which employs a cable for communication between sensing devices located near the drill bit and an intermediate communications system that is first mounted at the top of the drill string when a round-trip drill bit change is implemented. As drilling proceeds, drill pipes having an insulating coating painted thereon are added to the string, so that the intermediate communications system will eventually be a few hundred feet below the earth's surface. Rubber drill collar protectors are provided to prevent the drill pipe from rubbing against the casing. Communication between the intermediate communication system and a surface communications system is wireless. A toroidal antenna at the intermediate communications system launches a signal that is received by a toroidal antenna at the surface, the toroidal antenna surrounding a conductor that is connected between structure coupled to the drill string and the metal borehole casing. (Alternatively, the patent notes, potential between the drill string and the casing can be utilized.) The wireless link can be utilized for two-way communication, and can also be used for sending power downhole for operation without a battery or for charging a battery. The patent states that an important feature of the invention is to have the intermediate communication system away from the drill bit environment, and also indicates that the communication between the intermediate communication system and the surface is practical over only relatively short distances, for example, 1000 feet. Among the practical limitations of the apparatus described in this patent are the need for a cable between the intermediate communications and the system near the bottom of the hole, the need for providing an insulating coating on the upper portion of the drill string, and the limitations on the length of the wireless communication.
Other measurement while drilling schemes, communication systems, and control systems, are described in the following U.S. Pat. Nos. 2,225,668 2,354,887 2,400,170 2,414,719 2,492,794 2,653,220, 2,940,039 2,989,621 2,992,325 3,090,031 3,315,224 3,408,561 3,495,209 3,732,728 3,737,845 3,793,632 3,831,138 3,967,201 4,001,773 4,087,781 4,160,970 4,215,425 4,215,426 4,215,427 4,226,578 4,302,757 4,348,672 4,387,372 4,496,174 4,525,715 4,534,424 and 4,578,675.
Whereas a variety of wireless communication systems have been proposed for measurement while drilling, there has been a dearth of viable proposals for wireless communication in a cased borehole in which tubing is in place, and in which perforation, testing, stimulation, and/or production are typically to be implemented. The prospect of having wireless downhole communications in such a system, which can be used to communicate information in almost real time, and over relatively long periods of time, would appear to be a difficult objective. This is especially true if it is desired to have the system be operative to communicate with reasonable accuracy and data rate during operations which exacerbate the already hostile downhole conditions; for example, testing, stimulation, etc. These operations can involve severe pressure, temperature, and mechanical vibrations in the downhole environment and uncontrolled motion of the tubing.
It is among the objects of the present invention to provide a wireless communication system and method for use in a cased borehole that has been equipped with tubing. It is among the further objects of the invention to provide such a communications system which can operate under adverse conditions, including conditions that severely perturb the transmission path for communication; which can provide two way wireless communication between the earth's surface and one or more downhole locations; which is capable of communicating power to a downhole location, where the power is converted to a form suitable for use in operating the downhole subsystem or for storage for later use for such purpose; and which employs a coding scheme that permits accurate transmission of data, and which can be adapted for changes in the characteristics of the transmission path during particular conditions.